1. Field of the Invention
The present invention relates, in general, to fluid bearings and, more specifically, to air bearing for supporting moving webs or substrates.
2. State of the Art
Moving webs or substrates, such as paper, cellophane and foil that have printing formed thereon as the result of a printing operation, such as a web offset lithography, are guided and supported in a continuous path which may contain turns ranging between 0.degree. and 180.degree.. The direction of movement of the web is changed to align the continuously moving web with devices that fold, cut or perforate the final product. In addition, a 180.degree. re-orientation of the moving web to reverse the sides of the web (the bottom side becomes uppermost) and allow both sides of the web to be printed without a "perfecting" printing press which prints both sides simultaneously is sometimes desirable.
It is necessary to support the web a short distance above any bearing to prevent smearing of the fresh ink disposed on the web. A typical fluid or air bearing employed in web offset lithography is in the form of a hollow, chromed metallic cylinder having apertures arranged in one or more spaced rows in the peripheral side walls thereof. A hollow bore is formed in the cylinder and is connected to an air source such that air flows through the bore and the apertures in the side walls of the cylinder outward from the cylinder to create a thin cushion of air surrounding the cylinder. The cushion of air extends a short distance above the periphery of the cylinder to frictionlessly support and guide the web about the roller without contact between the web and the bearing.
One or more spaced rows of apertures are formed in the cylinder depending upon how much surface area of the web or substrate passes over the cylinder. Typically, the apertures in the cylinder result in uneven air distribution across the width of the web. Each high point is located above an aperture in the cylinder and low points are formed between two spaced apertures in the cylinder. The web occasionally contacts the cylinder at the low points which smears the fresh ink deposited on the web by the printing process.
Various attempts have been made to overcome these problems with previously deviced air bearings for use in web processes. Such attempts utilize rollers or bearings formed of porous, air permeable materials as disclosed in U.S. Pat. Nos. 3,245,334 and 3,744,693. In U.S. Pat. No. 3,245,334, a cylindrical member formed of a porous, air permeable material is stationarily mounted in a chamber through which a web passes. Air injected through a bore in the cylinder flows through the cylindrical member and forms a cushion of air a short distance above the periphery of the cylinder to support the web above the cylinder as the web moves past the cylinder. In one embodiment of U.S. Pat. No. 3,744,693, a stationarily mounted, hemispherically shaped turning bar is provided with a plurality of spaced apertures which form air flow paths for air supplied to the interior of the turning bar. A flexible sheet of a porous material is stretched and clamped under tension over the periphery of the turning bar to disperse air flowing through the apertures in the turning bar evenly about the periphery of the turning bar.
However, in both of the above-described devices, the air flow cylinder or turning bars stationarily mounted. This provides an even distribution of air only over a portion or one side of the cylindrical member and does not provide a full 360.degree. cushion. This restricts such air bearings to applications where the web is redirected less than 180.degree. from its original path.
Thus, it would be desirable to provide an air bearing which overcomes the above-identified problems encountered with previously deviced air bearings. It would also be desirable to provide an air bearing which is simple in construction. It would also be desirable to provide an air bearing which is usable with conventional, apertured air cylinders in new and retrofit applications. Finally, it would be desirable to provide an air bearing which provides a full 360.degree. air cushion about the periphery of the bearing to enable the air bearing to redirect moving webs more than 90.degree. from their original path.